# Dissociation constants of oxalic acid

For oxalic acid, $\mathrm{K}_{\rm a1}> \mathrm{K}_{\rm a2}$.

My teacher told me that after first deprotonation, $\ce{COO-}$ acts as an electron donating group and hence reduces the acidic strength of $\ce{HC2O4-}$ (due to the +I effect of $\ce{COO-}$ on the second $\ce{COOH}$).

Why is $\ce{COO-}$ an electron donating group when $\ce{COOH}$ is an electron withdrawing group?

• 1.25, 4.14 according to en.wikipedia.org/wiki/Oxalic_acid suggests it may be even slightly withdrawing. – Mithoron Oct 6 '15 at 19:46
• After loosing $\ce{H+}$ wont it become rich in electrons and act as an $\ce{+I}$ group ? – Sujith Sizon Oct 15 '15 at 12:22